Low temperature drying and heating apparatus of known construction utilizes enormous quantities of fuel which ordinarily is in the form of natural gas or fuel oil, prices of which have increased substantially in recent years.
Combustion of all liquid or solid fuels requires that the fuel be gasified before being burned. In the case of a liquid fuel, the fuel is normally atomized to reduce particle size. Most heat transfer within the flame is by radiation with convection playing a minor role. However, in either case surface area plays a major part in heat transfer and in the rate of evaporation. Burning time of a particle is dependent on the rate of evaporation or gasification.
In the case of solid fuels, gasification of the volatile portions of the solid fuel act much like the gasification of atomized fuels. However the carbon or solid portion must be heated so that carbon on the surface of the coal particle combines with oxygen forming carbon monoxide which burns. Again particle size, i.e., surface to mass ratio, are very important factors which control burn time.
Intimate mixing of coal particles with air is also another critical factor in the rate of reaction and the burning time of pulverized coal. Conventional coal burners introduce pulverized coal in rather concentrated streams which mix slowly thereby producing relatively long flames having slow reaction rate.
Rate of reaction is effected by the temperature of the reaction. Therefore it is advisable to maintain high temperatures in the reaction zone of any furnace, oven or dryer. Unfortunately known low temperature ovens or furnaces do not lend themselves to providing the necessary high temperature combustion zone required to burn coal efficiently.